Fires, Hurricanes, and Volcanoes: Comparing Large Disturbances
Author(s): Monica G. Turner, Virginia H. Dale, Edwin H. Everham, III
Source: BioScience, Vol. 47, No. 11 (Dec., 1997), pp. 758-768
Published by: University of California Press on behalf of the American Institute of Biological Sciences
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I
Fires,
Hurricanes,
and
Comparing
Large
Volcanoes:
Disturbances
Largedisturbancesare heterogeneousspatiallyand not as
catastrophicas theymay initiallyappear
Monica G. Turner, Virginia H. Dale, and Edwin H. Everham III
T
he importance of natural disturbances in shaping landscapes and influencing ecosystems is now well recognized in
ecology (e.g., Pickett and White 1985,
Turner 1987, White 1979). Disturbance can be defined generally as
any relatively discrete event in time
that disrupts ecosystem, community,
or population structure and changes
resource or substrate availability or
the physical environment (White and
Pickett 1985). In recent years, ecologists have learned a great deal about
the dynamics and effects of relatively
small, frequent disturbances. ExtenMonicaG.Turner(e-mail:[email protected].
edu)is an associateprofessorof terrestrial
ecology in the Departmentof Zoology at
the Universityof Wisconsin,Madison,WI
53717. Her researchfocuseson the causes
and consequencesof spatial heterogeneity, especiallyat landscapescales, and she
has been studying the Yellowstone fires
since 1988. Virginia H. Dale (e-mail:
[email protected])is senior scientist in the
EnvironmentalSciences Division at the
Oak Ridge National Laboratory, Oak
Ridge, TN 37831-6035. Her research
emphasizes forest ecology, disturbance
dynamics,and land-use change, and she
has been trackingsuccessionaldynamics
followingthe 1980 eruptionof Mount St.
Helens. Edwin H. EverhamIII (e-mail:
is an assistant [email protected])
fessor of environmental studies in the
Collegeof Artsand Sciencesat the Florida
Gulf Coast University, Fort Myers, FL
33908-4500. His researchfocuses on the
role of disturbances,naturaland anthropogenic,in tropicalforestecosystemsand
has emphasizedthe effects of hurricanes
and windstorms.? 1997 AmericanInstitute of BiologicalSciences.
758
II II
Whether large
disturbances are
qualitatively different
from numerous small
disturbances remains
an unresolved issue
in ecology
sive studies addressing patch dynamics and gap-phase replacement (where
a canopy tree has died, initiating
active recruitment of new individuals into the canopy), especially in
temperate deciduous and tropical forests, have led to good understanding
of and predictive capability in these
systems (e.g., Runkle 1985). By contrast, natural disturbances that affect large areas but occur infrequently
have not been well studied. Whether
large disturbances are qualitatively
different from numerous small disturbances remains an unresolved issue in ecology, in part because of a
paucity of long-term data on the
effects of large-scale disturbances and
the impossibility of replicating such
events.
During the past two decades, several large natural disturbances-the
eruption of Mount St. Helens in 1980,
the Yellowstone fires of 1988, and
Hurricane Hugo in 1989-captured
the attention of the public and received considerable postdisturbance
ecological research. In this article, we
compare these three natural disturbances to determine whether general
trends in the ecological effects of
large-scale disturbance can be detected. For example, do large disturbances create similar kinds of heterogeneity across the affected landscape?
Are the spatial patterns of disturbance predictable based on landscape
position? Are recovery mechanisms
similar? We focus on similarities and
differences in the characteristics of
the disturbances, in the effects of the
disturbances on vegetation, and in
the postdisturbance recovery of vegetation. Our analysis takes advantage of existing data for each disturbance and draws on our individual
experiences in each ecological system. Because the independent studies were not designed comparatively,
data for quantitative comparisons
are not always available. Nevertheless, we believe that these contrasts
may provide a foundation for a detailed comparative, synthetic analysis.
The three large disturbances
We begin by providing a brief introduction to the three disturbances to
be compared. These events were high
in intensity and large in spatial extent.
Mount St. Helens. Mount St. Helens
is an active volcano in southwest
Washington. Before 1980, the mountain was surrounded by a patchwork
of forested and clear-cut land in varying stages of reforestation. The 18
May 1980 eruption affected more
than 700 km2 and created a variety
of disturbances in the immediate vi-
BioScience Vol. 47 No. 11
cinity of Mount St. Helens: a 0.25
km2 pyroclastic flow; a 550 km2 area
of trees that had been blown down
bordered by 96 km2 of scorched trees;
a 60 km2 debris avalanche; and massive mudflows (Figure 1). The interaction of disturbance characteristics
and the life forms of species present
at the time of the eruption resulted in
different patterns of surviving plants
in these areas (Adams et al. 1987,
Franklin et al. 1985).
The pyroclastic flow consisted of
hot gases, rock fragments, and superheated steam, which poured forth
from the crater at temperatures of
350-850 ?C. No plants survived on
these flows, and the slow recovery is
characterized by diverse herbaceous
and forb species whose seeds were
transported by wind (del Moral and
Bliss 1993). The blowdown zone included areas where the eruption force
was strong enough to knock over
trees, although herbaceous and understory vegetation survived, particularly in sites buried under snow
(Franklin et al. 1985, Halpern et al.
1990). Temperatures of materials
deposited in the blowdown zone
ranged from 100 to 350 ?C. Encircling the blowdown zone was the
scorch perimeter, where temperatures were hot enough to burn leaves,
but winds were not strong enough to
down the trees. Coniferous treeswhich do not have the ability to
but
releaf after defoliation-died,
some deciduous trees survived
(Adams et al. 1987).
The debris avalanche deposits, or
landslides, were a relatively cool 95
?C but were deep, averaging 45 m
but reaching depths of 150 m in
some locations (Fairchild 1985). No
viable seeds survived the landslide;
most of the early successional species that colonized the avalanche have
plumed seeds that can disperse long
distances (Dale 1989). The very few
surviving plants developed from
rootstocks or stems that were transported by the landslide and came to
rest near the surface (Adams et al.
1987). Vegetation cover on the debris avalanche gradually increased
from 0 to 35% by 1994. A few conifers have established, but red alder
(Alnus rubra) now has the greatest
cover. Even so, the distribution of
red alder is patchy, and it is absent
from large areas.
December1997
1.
Figure1. Disturbanceeffectscreatedby the 1980 eruptionof Mount St. Helens. This
volcanic eruption created several distinct disturbances,including the crater itself,
pyroclasticflow, the debrisavalanche,mudflows,blowdown,and singeperimeterand
ash deposits.
.
--
-~
......ds
ceu ....*p
able
tgr
Figure 2. The 1988
Yellowstone fires created a strikingmosaic
of burnseveritiesacross
the landscape. Blackened areasindicateareasof crownfire,brown
sections show areasof
severesurfaceburns,
and green forest indi-
cates eitherlight surfaceburnsor unburned
forest.
Mudflows occurred on the major
rivers draining the Mount St. Helens
area, induced either by earthquakegenerated liquefaction (i.e., numerous earthquakes shook the waterladen debris avalanche so severely
that water rose to the surface) or by
the rapid melting of glaciers on the
mountain. Although much of the
understory vegetation and small trees
were washed away by the mudflows,
large trees that were taller than the
surface of the flow survived. The
proximity of surviving vegetation and
seeding by humans (often involving
exotic graminoids) have resulted in
close to 100% cover on the mudflows today (Halpern and Harmon
1983).1
The eruption also deposited tephra (i.e., solid material ejected durWV.H. Dale, unpublishedobservation.
ing the eruption and
transported through
the air) over thousands of square kilometers. Trees
survived in these areas, but herbaceous plants were buried under deep
layers of tephra. Some plants were
able to grow through as much as 15
cm of material (Antos and Zobel
1985). Where tephra was less than 25
cm deep, both survival of buried
herbaceous vegetation and establishment of seedlings were enhanced by
the burrowing of pocket gophers
(Thomomystalpoides;Andersonand
MacMahon 1985).
The 1988 Yellowstone fires. Yellowstone National Park encompasses
9000 km2 in the northwest corner of
Wyoming and is primarily a high,
forested plateau. Fire has long been
an important component of this landscape (Romme 1982, Romme and
Despain 1989), and, as in most other
parts of the Rocky Mountains, fire
759
has profoundly influenced the fauna,
flora, and ecological processes of the
Yellowstone area (e.g., Despain
1991, Houston 1973, Taylor 1973).
A natural fire program that was
initiated in 1972 permitted lightningcaused fires in Yellowstone National
Park to burn without interference
of
under prescribed conditions
weather and location. Most of the
235 fires observed in the park between 1972 and 1987 went out by
themselves before burning more than
1 ha, and the largest fire (in 1981)
burned approximately 3100 ha.2
Thus, the size and severity of the 1988
fires, which burned over 250,000 ha
in the Greater Yellowstone region due
to prolonged drought and high winds
(Renkin and Despain 1992), surprised many managers and researchers. The 1988 fires also spread rapidly through all forest successional
stages and were influenced more by
wind speed and direction than by
subtle patterns in fuels or topography. Reconstructions of fire history
suggest that the last time a fire of this
magnitude occurred in Yellowstone
was in the early 1700s, so the 1988
fires may represent a major disturbance event that occurs at intervals
of 100-300 years in this landscape
(Romme and Despain 1989).
The Yellowstone fires created a
strikingly heterogeneous mosaic of
burn severity (Figure 2) across the
landscape (Turner et al. 1994). In
the most severely burned areas, where
crown fires (fires that consume and
spread through the tree crowns) occurred, needles of the canopy trees
were completely consumed by fire,
and tree mortality was essentially
100%. The soil organic layer was
almost entirely burned, leaving the
soil bare, with no litter. However,
because the depth to which soil was
charred averaged only 13.6 mm in
crown fires, resprouting of herbaceous plants and shrubs was significant (Turner et al. 1997). Other areas were affected by severe surface
burns, in which canopy tree mortality was extensive but the conifer
needles were not consumed by the
fire. In such areas, the soil organic
layer that existed before the fire was
largely burned, but the soil was soon
2D. Despain, 1990, personal communication.
National Biological Service, Bozeman, MT.
760
covered by dead needles that had
fallen from the canopy. Still other
areas were affected only by light surface burns, in which the canopy trees
retained green needles and generally
did not die, although their stems
often were scorched. The soil organic layer of these areas remained
largely intact, although small areas
(on. the order of several square
meters) were patchily burned. The
percent cover of herbaceous vegetation in these light surface burns was
not distinguishable from that in unburned forest within two years following the fires (Turner et al. 1997).
Hurricane Hugo. Hurricanes are an
important force influencing forest
composition and structure on numerous islands and coastal regions
(Waide and Lugo 1992, Weaver
1986). During the past 300 years, 15
hurricanes have passed over the island of Puerto Rico (Scatena and
Larsen 1991). Hurricane Hugo
passed over the northeast corner of
Puerto Rico on 18 September 1989
as a category 4 hurricane with sustained winds of over 166 kph (Boose
et al. 1994, Scatena and Larsen
1991). Although Hugo continued to
the northwest and struck the coast of
South Carolina, where it affected
over 3 million hectares of timberland (Sheffield and Thompson 1992),
we focus here only on the impact of
the hurricane in Puerto Rico, particularly the 11,000 ha Luquillo Experimental Forest in the northeast
corner of Puerto Rico. Hurricane
Hugo passed to the northeast of the
Luquillo Experimental Forest on a
northward track, generating winds
that came first from the northeast
and then, with the passage of the
storm, shifted to the north and northwest (Scatena and Larson 1991).
Prior to Hurricane Hugo, the last
hurricane to have a direct impact on
the Luquillo Experimental Forest was
San Ciprano in 1932, although San
Felipe (in 1928), San Nicolas (in
1931), and Santa Clara (also known
as Betsy, in 1956) had tracks near
the Luquillo Experimental Forest and
may have had some localized effects
on it (Weaver 1986).
Hurricanes are an important and
possibly the dominant influence on
forest composition and structure in
the Luquillo Experimental Forest
(Waide and Lugo 1992, Weaver
1986). The forest canopy consists of
trees and fewer
small-crowned
emergents than continental tropical
forests (Weaver 1986). Simulation
studies indicate that patterns of diversity and dominance in this forest
are maintained by regular hurricane
disturbance. In the absence of hurricanes, the forest becomes increasingly dominated by late-successional
species, and diversity begins to decline
after 60 years (Doyle 1981); these
trends are supported by long-term studies of posthurricane forest dynamics
earlier this century (Weaver 1986).
Hurricane disturbance includes
both wind and rainfall effects. Wind
intensity may be quantified by maximum gusts, sustained wind speed,
duration of wind, and distance from
the center of the storm system
(Everham and Brokaw 1996). Scatena and Larsen (1991) developed
three indices of storm intensity: maximum sustained wind and storm duration, maximum sustained wind and
proximity to the storm center, and
rainfall totals (as a percentage of
annual rainfall). Hugo, with a fourhour duration over the island and a
maximum rainfall of 339 mm (15%
of the annual total), was classified as
a moderate-intensity event (Scatena
and Larsen 1991). As with the eruption of Mount St. Helens and the
Yellowstone fires, disturbance intensity varied over the landscape (Scatena and Larsen 1991), leading to a
mosaic of effects (Figure 3). However, detailed information on the spatial variation of wind and on the rain
intensity is not usually available after a hurricane because of the impact
of wind on remote weather stations.
Hurricane wind alters the structure of the forest canopy, reducing
canopy height by an average of 50%
(Brokaw and Grear 1991). The severity of wind damage to vegetation
ranges from defoliation to debranching, stem breakage, and uprooting.
Mortality of trees following hurricanes tends to be low, rarely exceeding 40% (Everham and Brokaw
1996). The severity of hurricane-related wind damage in the Luquillo
Experimental Forest decreased from
the northeast to the southwest. This
pattern of wind damage was related
to proximity to the storm's center
and was modified by the Luquillo
BioScience Vol. 47 No. 11
Table 1. Comparison of characteristics of three broad-scale disturbances.
Characteristic
Crown fires
Hurricanes
Volcanoes
Disturbance-generated heterogeneity
Coarse-grained mosaic with some
fine-grained variation
Fine-grained mosaic
Very coarse grained landscape
mosaic
Endogenous versus exogenous
Both: quality and quantity of fuel
plus synoptic weather
Exogenous
Exogenous
Spatial predictability of disturbance
pattern
Unpredictable because topography
has little effect under severe burning
conditions
Predictable based on terrain
at broad (tens to hundreds
of hectares), but not fine,
(less than 1 ha) scales
Predictable based on direction of
blast and topography
Return time of disturbance in
the landscape
100-500 yr, depending on latitude
and elevation
60-200 yr in Puerto Rico,
depending on intensity of
storm; varies geographically
100-1000 yr for "ring of fire"
volcanoes around the Pacific
Ocean; Mount St. Helens last
erupted in 1857
Seasonality of disturbance
Summer
Summer and fall
None, although timing of
eruption will influence recovery
Mountains. Trees on the southwest
slopes of the mountains suffered
fewer broken or uprooted stems, with
canopy damage limited mainly to
defoliation and debranching. In addition, there were fewer broken or
uprooted stems on the northwest side
of the mountains (4-20% of stems in
study plots) than on the northeast
side (11-49 % of stems in study plots).
Gaps created by damage to the canopy
varied from smaller than 0.0025 ha
(associated with branch damage) to
multiple treefall gaps, which can exceed 0.04 ha (Everham 1996).
The rainfall associated with hurricanes often triggers landslides.
Landslides ranging in size from 0.002
to 0.482 ha occurred in 9.1 ha of the
6474 ha of Luquillo Experimental
Forest surveyed with aerial photography after Hurricane Hugo (Scatena
and Larsen 1991). These landslides
were associated with steep concave
slopes and with the heavier rainfall
that occurred closer to the center of
the storm. Recovery of vegetation on
landslides was faster in the lower
zone of accumulation than in the
upper area of the slide, but even
these areas may return to predisturbance forest conditions within 50
years (Guariguata 1990).
Characteristics of
the disturbances
Disturbances and disturbance regimes can be described with a variety
of parameters (e.g., White and Pickett
1985). We focus on five: the landDecember 1997
scape heterogeneity created by the
disturbance; whether the disturbance
is exclusively exogenous to the system or incorporates some endogenous
factors; whether the spatial pattern of
disturbance was predictable based on
landscape position; anticipated return
time; and any regular seasonality in
the disturbance events.
All three disturbances exhibited
substantial heterogeneity in their severities across the landscape, and the
spatial scale of the disturbance mosaic varied (Table 1). In Yellowstone,
the mosaic was of differential fire
severity, and islands of unburned
forest remained within the overall
burn perimeters (Christensen et al.
1989, Turner et al. 1994). Although
some areas of crown fire extended
for several kilometers, 75% of the
most severely burned areas were located within 200 m of unburned or
lightly burned forest (Turner et al.
1994). Hurricane Hugo created a
mosaic of disturbance effects (winddriven defoliation, blowdowns, and
landslides) that varied in type more
than effects observed in Yellowstone;
each type could, in turn, be characterized by levels of severity (e.g.,
percentage of individuals or area affected). The spatial pattern of disturbance effects in the landscape was
not quantified as was done for the
Yellowstone fires, but the mosaic
resulting from Hurricane Hugo appeared to be more fine grained than
that resulting from the Yellowstone
fires-that is, it varied over tens of
meters rather than thousands of
meters. Estimates of the spacing of
hurricane-created gaps ranged from
10 to 35 m and varied with the
method of quantifying damage; regardless of how damage was measured, the mode of patch size was
0.0025 ha (Everham 1996).
The volcanic eruption of Mount
St. Helens also led to substantial
landscape heterogeneity. As was the
case with Hurricane Hugo, disturbance effects varied qualitatively
across the landscape (i.e., debris avalanche, mudflows, pyroclastic flows,
blowdown, crater formation, and ash
deposit). Of all three disturbances,
the Mount St. Helens eruption probably had the greatest diversity of
types of disturbance effects. Again,
severity varied within each type of
effect. The mosaic that was created,
however, was coarse grained, with
large, isolated patches and linear
mudflows. Thus, among the three
disturbances, the effects at Mount
St. Helens exhibited the most diversity and those at Yellowstone the
least, and the disturbance-created
mosaic was most coarse grained at
Mount St. Helens and most fine
grained in Puerto Rico following
Hurricane Hugo (Table 1).
The predictability of the spatial
pattern of disturbance varied among
the three disturbances (Table 1). For
the Yellowstone fires, the spatial
pattern of disturbance was generally
not predictable based on landscape
position: Analyses based on slope,
aspect, and vegetation pattern did
not explain variability in fire pattern
761
Figure3. Spatialheterogeneityof hurricanedisturbanceeffectsinPuerto Rico, as shown from
the top of the Mt.
Britton tower in the
LuquilloExperimental
Forest,six monthsafter
HurricaneHugostruck.
Multiple treefalls created large patches of
severe damage within
standsthatweredefoliatedbutalreadyleafing
out.Photo:AllanDrew.
(Turner et al. 1994), and the
addition of topographic effects in a spatial model of
fire spread (Gardner et al.
1996) did not substantially
alter simulated fire patterns.
Under burning conditions as
severe as those of the Yellowstone fires, fire spread is
controlled almost wholly by
wind direction and speed
rather than by landscape
variation (Bessie and Johnson 1995). Even large geographic features, such as the
Grand Canyon of the Yellowstone, were not effective
as fire breaks.
By contrast, a combination of topographic position
(i.e., slope, elevation, and
aspect) and knowledge of the
direction of the storm or
blast could be used to predict regions of maximum or
minimum exposure for both
Hurricane Hugo and the
Mount St. Helens eruption
(Table 1). The disturbances
created by the eruption of
Mount St. Helens were due
more to characteristics of the
volcanic eruption than to topography. Because of a
weakness in the layer of rock
on the north side of the
mountain, the eruption was
a northward-directed blast
that left the south side of
the mountain virtually unFigure 4. Recovery of the hurricane-damaged touched.
However, the pycanopy showed rapidrefoliation and restructuring
roclastic
flow
and blast were
of the canopy, as illustratedin this view of the Rio
Mamayes, a river in the Luquillo Experimental forceful enough to overcome
Forest (top) six months, (middle) 18 months, and topographic features in the
(bottom) 30 months after Hurricane Hugo. Pho- vicinity of the mountain. The
tos: Allan Drew.
mudflows followed the river
762
valleys but defined new channels.
For Hurricane Hugo, the Luquillo
Mountains provided some protection at the landscape scale from the
predominantly north and northwest
winds (Boose et al. 1994). Southwest
slopes of the mountains were least
affected by the wind. Valleys also
provide protection if they are oriented away from the wind; in valleys
oriented parallel to the wind, by contrast, the increased velocity of the
channeled wind may cause severe
damage (Everham 1996, Everham
and Brokaw 1996).
Hurricanes and volcanoes are
largely exogenous disturbances, in
which the existing vegetation has
little or no influence on disturbance
intensity and pattern. However, local wind gusts during hurricanes can
be affected by canopy structure;
moreover, stand age, density, and
successional class may influence severity of damage (Everham and
Brokaw 1996, Zimmerman et al.
1994). Both exogenous (synoptic
weather conditions; e.g., Bessie and
Johnson 1995, Johnson and Wowchuk 1993) and endogenous (fuel
availability) components affect fire
intensity and pattern. Provided that
fuel is available, however, large
crown fires are not greatly influenced by variations in fuel structure
that are associated with successional stage. Thus, each of the three
disturbances is primarily exogenous.
Return times for each disturbance-that is, the interval between
recurrent disturbances at a given location-are relatively long, ranging
from decades to centuries for large
hurricanes and from one to several
centuries for crown fires and for
volcanoes (Table 1). Fires and hurricanes both tend to occur seasonally
from summer through early fall,
whereas volcanic eruptions occur
without any seasonal regularity.
However, the timing of an eruption
will strongly influence the effects on
vegetation. An eruption during the
growing season, for example, would
have different effects than one during a season when plants are dormant. Even in the wet tropical forests of the Luquillo Experimental
Forest, seasonal patterns of flowering and fruiting (Lugo and Frangi
1993) can influence the effects of a
specific hurricane.
BioScience Vol. 47 No. 11
Table 2. Effects of large-scale disturbances on vegetation.
Effect on vegetation
Crown fires
Hurricanes
Volcanoes
Differentialdirectmortalityamong Generallynot, at least amongtrees in Greatermortalityof early suc- Differentialmortalityrelatedto
speciesor life forms?
Yellowstone;mortalityoften 100% in cessionalspecies;exposureto the positionof the dormantbuds,
wind increaseswith stem size but mortalityis often more
stand-replacingburn
than 99%
Is there extensive delayed mortality? No; however, some scorched trees may Yes; mortality increased up to No
If so, at what time lag?
die or be more susceptible to other
50% from years 1 to 3; delayed
disturbance
mortality may last for more
than five years
Preconditioning of vegetation response by other disturbance types?
No; however, previous fire events may
have an influence
Effects of disturbance on the
plant community
One effect of a large disturbance is
that it can directly kill individual
plants. All three disturbances did so,
but the patterns and timing of mortality varied (Table 2). In Yellowstone,
data on direct mortality are available
only for tree species. The coniferous
forests of Yellowstone are dominated
by lodgepole pine (Pinus contorta
var. latifolia), although subalpine fire
(Abies lasiocarpa), Engelman spruce
(Picea engelmannii), aspen (Populus
and Douglas fir
tremuloides),
(Pseudotsuga menziesii) may be locally abundant. Differential mortality among species was not observed,3
and tree mortality was 100% in
crown fires and severe surface burns.
However, in less severe surface burns
tree mortality was related to tree size,
with larger trees (e.g., 15-20 cm diameter at breast height [dbh]) being less
likely to die than small trees.
Tree mortality following Hurricane Hugo was related to successional stage and to height within the
canopy. Early successional species
(e.g., Cecropia schreberiana) tended
to undergo stem snap (21.3% of
stems) and death (52.9%). Late successional species (e.g., Dacryodes
excelsa) tended to lose branches
(29.9% of stems); both stem snap
(3.6% of stems) and mortality (1.6%)
were low in these species (Zimmerman et al. 1994).
differences in
Species-specific
damage and mortality obscured relationships to size or height within
3M. G. Turner, unpublished data.
December 1997
Yes; chronic wind stress can
precondition, although its impacts differ with direction of
chronic wind (e.g., trade
winds) and of a hurricane
species. Whereas Frangi and Lugo
(1991) found greater structural damage to intermediate-sized stems (1214 m in height) in the Luquillo Experimental Forest, Basnet et al.
(1992) reported that intermediatesized stems (20-25 m in height, 5060 cm dbh) were the least damaged.
A positive correlation between stem
size and damage has been suggested
from studies of other hurricanes in
New England (Foster and Boose
1992), Puerto Rico (Wadsworth and
Englerth 1959), the Solomon Islands
(Whitmore 1974), and Mauritius
(King 1945). However, survival was
highest among the largest trees following a hurricane in Nicaragua
(Boucher 1990). A thorough review
of impacts of intense wind disturbances suggests a more complex relationship between stem size and
damage (Everham and Brokaw
1996). The smallest stems are sheltered from the wind but are subject
to indirect damage from other falling stems. Exposure to wind increases
as stem size increases, but the largest
stems may be preconditioned by previous wind and survive subsequent
disturbance.
On Mount St. Helens, plant mortality often exceeded 99% of individuals in severely affected areas,
with few differences in mortality
among species. Patterns of survival
were related to the position of dormant buds (Adams et. al. 1987).
Plants with subterranean dormant
buds (geophytes) and those that could
generate from fragments best survived the blast, scorch, and landslide. Survival on the mudflow was
higher for large trees.
Yes; adaptations to fire, snowslides, and recurrent floods may
reduce effects of volcano
Most disturbance-induced plant
mortality was immediate for the
crown fire and volcano, but hurricane-caused plant mortality showed
a time lag of several years (Table 2).
In Yellowstone, although most mortality was immediate, some scorched
trees near the perimeter of the burned
area likely died the following year or
could have been stressed sufficiently
to be more susceptible to other disturbances (Knight 1987). At Mount
St. Helens, some trees on the mudflow died later (depending on the
depth of the mud), but again, most
mortality was immediate. Following
Hurricane Hugo, by contrast, mortality increased by as much as 50%
between years one and three, indicating time lags of several years
(Everham 1996, Walker 1995). Mortality following a hurricane may be
delayed for as long as five years, as has
been observed in Jamaica (Bellingham
1991), Florida (Craighead and Gilbert
1962), and Sri Lanka (Dittus 1985).
We also investigated whether prior
exposure to different stresses or disturbances might "precondition" vegetation such that it can better recover from a large disturbance. For
crown fires, other types of disturbance do not appear to influence
responsiveness to fire, although fire
history may be important. For example, lodgepole pine (Pinus contorta
var. latifolia) is well known for having serotinous cones that release their
seed when heated. Prefire serotiny
levels strongly influence postfire succession, and Muir and Lotan (1985)
found that the time and severity of
the most recent fire at a site was the
best predictor of percent serotiny in
763
Table 3. Postdisturbance recovery of the vegetation.
Recovery property
Crown fires
Hurricanes
Primarymodes of recovery
Dispersaland seedlingestablishment
(for dominanttrees);resprouting
(for herbsand shrubs)
Resprout,seedlingestablishment,Dispersaland seedlingestablishor releasefrom suppressedstage ment (afterthe crater,pyroclastic
(for dominanttrees;not quanti- flow, and debrisavalanchedisfied for herbsand shrubs)
turbances)
Seedlingdynamics
Earlypulse (1-3 yr postfire)
Earlypulse (1.5-2.5 yr after
hurricane)
No pulse;establishmentgradual
Reestablishmentrate
Percentcover reestablishedrapidly
Very rapid
Slow, althoughspatiallyvariable
Plantspeciesdominance
Shiftsin herbaceousspeciesbut not
trees;landscapediversitylikely to
increase
Shiftswith largepulse of early Shiftswith succession;landscape
successionalspecies;landscape diversitymay increase
diversitymay increase
Plantspeciesrichness
Maintainedat landscapescale,
increasinglocally
Maintainedat landscapeand
local scales
lodgepole pine. Thus, stands that
have burned more frequently in the
past may be reforested more rapidly
following a crown fire.
For hurricanes and volcanoes,
however, some preconditioning may
occur from other disturbances.
Chronic wind stress may strengthen
limbs and alter vegetation architecture such that effects of a hurricane
are less severe (Webb 1958). However, the actual impact of a given
hurricane will depend on whether
the direction of the chronic wind
(e.g., trade winds) and of the hurricane coincide. In addition to morphological changes that mitigate severity
of damage, response to hurricane
disturbance may be facilitated by
adaptations of species to the frequent treefalls associated with background tree mortality. Similarly, for
Mount St. Helens, adaptations to
fires, snowslides, and floods appear
to have reduced the effect of the
volcano (Adams et al. 1987). Periodic fires and storms (in particular,
heavy precipitation, mainly in the
form of snow) may explain the high
proportion of geophytes in the Pacific
Northwest flora. Montane forest and
subalpine meadows of Washington
have recurrent fires and snowslides,
which remove aboveground plant biomass in a manner similar to that of
the 1980 lateral blast of Mount St.
Helens. Species with the ability to
regenerate from meristematic tissue
located several centimeters below the
surface have the greatest survival
rate during fires (Flinn and Wein
1977). The species that regenerated
764
from underground buds in the blast
zone and scorch areas of Mount St.
Helens also regenerate after fires and
snowslides (Cushman 1981).
Postdisturbance
vegetation recovery
What is the legacy of a large-scale
disturbance? Disturbance effects are
mitigated by the ability of the system
to recover, and the mechanisms of
vegetation recovery are quite varied.
We focus on modes of recovery in
trees, shrubs, and herbaceous species; whether seedling establishment
was gradual or pulsed; and how cover
and species richness has changed
through time (Table 3).
The mechanisms leading to vegetation reestablishment in Yellowstone
and Mount St. Helens appeared to
be similar; those in Puerto Rico were
more varied. For Yellowstone and
Mount St. Helens, reestablishment
of the dominant trees, which in both
cases are conifers, occurred only
through seed. By contrast, the tree
species of Puerto Rico exhibited varied recovery mechanisms, including
resprouting, seeding, or rapid growth
following hurricane-induced opening of the canopy (Figure 4). Although it is possible that the differences in recovery mechanisms reflect
the fact that Puerto Rico is a tropical
system, whereas the other two systems are temperate, both temperate
and tropical forests show a similar
complexity of responses following
intense wind disturbance (Everham
and Brokaw 1996). Reestablishment
Volcanoes
Increasingat landscapeand local
scales;exotics increasebut cause
native speciesto decline
of herbaceous species in areas of
Mount St. Helens where ash deposition was less than 25 cm deep was
similar to herbaceous reestablishment in areas of stand-replacing burn
(i.e., crown fire or severe surface
burn) in Yellowstone. In both systems, perennial grasses, sedges, and
forbs often resprouted and began
filling the disturbed area (Antos and
Zobel 1985) in a manner similar to
that observed following the 1912
eruption of Mount Katmai (Griggs
1919). Where ash deposition on
Mount St. Helens exceeded 25 cm,
resprouting did not occur, and colonization by seed was needed for plant
reestablishment (Dale 1989, Franklin
et al. 1985). The dynamics of herbaceous species were not quantified
following Hurricane Hugo.
Postdisturbance seedling establishment occurred in a pulse following the Yellowstone fires and Hurricane Hugo. In Yellowstone, new
conifer seedlings were abundant during the first two years following the
fires, and new aspen seedlings were
present only during the first postfire
year (Romme et al. 1997). Herbaceous species flowered profusely in
the burned forests in 1990 (Figure
5). Seedlings of herbaceous plants
peaked during 1991, three years after the fires, and then returned to
low levels (Turner et al. 1997). In
Puerto Rico, seedlings were most
abundant 1.5-2.5 years following the
hurricane, and seedfall was two to
three times greater than prehurricane
levels (Walker and Neris 1993). By
contrast, a seedling pulse was not
BioScience Vol. 47 No. 11
observed on Mount St. Helens during the first 15 years following the
eruption, although average plant
cover throughout the area has gradually increased (Figure 6). The area
affected by the volcano was so large
that dispersal distances exceeding
several kilometers were often needed
for vegetation establishment, and establishment has been gradual rather
than pulsed. On the mudflows, deciduous trees that established as seedlings provided 100% cover by 1994.
On the debris avalanche, cover approached 34% by 1994 but was extremely variable across the deposit.
Vegetation recovery on the pyroclastic flows is still minimal and consists
of isolated individuals or groups of
plants in favorable microsites (del
Moral and Bliss 1993).
Patterns of species richness varied
among the three disturbances. Plant
species richness was still increasing
at the scale of sampling plots (less
than 10 m2) five years after the Yellowstone fires, but overall species
richness of the landscape was unaffected by fire. That is, species richness varied locally and increased as
species "filled in," but cumulative
species richness across all sampling
points seemed insensitive to the fires.
However, dominance among the herbaceous species has shifted following fire, as many species recovered
but varied in relative abundance.
Fireweed (Epilobium angustifolium),
for example, became much more abundant after the fire than it was before.
Landscape diversity-the number and
relative abundance of different community types-may increase if areas
previously dominated by conifer forests shift to other community types,
such as meadows or aspen groves.
Following Hurricane Hugo, plant
species richness increased at some
sites at an intermediate spatial scale
(i.e., hectares) but was maintained at
both larger and smaller scales. As in
Yellowstone, however, species dominance shifted substantially because
early successional species thrive in
the hurricane-created gaps. The addition of these early successional
patches also may lead to a shortterm increase in landscape diversity.
Over the next three decades, selfthinning and the short life span of
the early successional species should
landscape diversity (Everham 1996,
Weaver 1986).
On Mount St. Helens, species richness was still increasing at the plot
level 15 years after the eruption. Of
296 species reported for the area
prior to the eruption (St. John 1976),
156 species occurred on the debris
avalanche by 1994,4 and 16 occurred
on the pyroclastic flows by 1990 (del
Moral and Bliss 1993). Species dominance is likely to shift as succession
proceeds, and new community types
may be introduced within the landscape. A complicating factor at
Mount St. Helens was colonization
by exotic species (sometimes resulting from intentional seeding of exotics), which resulted in local increases
in cover but decreased richness of
native species and increased mortality of tree seedlings (Dale 1991).
Finally, given our current conceptual understanding of postdisturbance
succession at each site, how well can
we explain vegetation recovery? Although the important factors controlling recovery vary among the
sites, two important similarities were
observed. First, at all three locations
a hierarchy of factors appears to
control succession, with different factors being more important at different spatial scales. In Yellowstone,
postfire succession is controlled by
broad-scale gradients in serotiny, by
meso-scale variability in fire severity
and patterning, and by microscale
variability in soil conditions, slope,
and aspect. Following Hurricane
Hugo, pathways of vegetation response were first related to broadscale patterns of severity of canopy
damage (Everham 1996). If mortality and structural damage were minimal, recovery occurred through
resprouting. Greater structural damage or mortality led to increased establishment of early successional
species. With either response, succession was also influenced by landscape patterns of light and moisture
availability. At Mount St. Helens,
disturbance type (especially depth of
volcanic material) determined the
degree of vegetation survival over
broad scales. Recovery was influenced largely by prevalence of seeds
being dispersed, which was a function of both distance to surviving
result in reduced dominance and
4V. H. Dale, unpublisheddata.
December 1997
vegetation and wind patterns (Dale
1991). Microtopography, local disturbances, and herbivory were important on a local scale (Dale 1989,
del Moral and Bliss 1993).
The second similarity was evidence
for multiple successional pathways
at each site and for the early development of certain communities that inhibit subsequent vegetation reestablishment. In some of Yellowstone's
burned forests, those in which the
percentage of serotinous trees was
low before the 1988 fires, the rapid
development of herbaceous vegetation may preclude tree establishment
for decades. In Puerto Rico, the development of fern meadows following the hurricane is also likely to
On
inhibit forest development.
Mount St. Helens, extensive lichen
development may inhibit tree establishment and forest development for
decades, as it has at the Kautz Creek
Mudflow on Mount Rainier (Frehner
1957, Frenzen et al. 1988). Thus, a
positive feedback process may occur
in which the lack of rapid plant establishment permits lichen establishment, a relatively slow process, and
the lichens then inhibit plant growth.
Synthesis:legaciesof
large disturbances
Ecologists still have relatively few
observations and long-term studies
of disturbance events that are large,
severe, and infrequent. The three
large-scale disturbances that we have
compared here shared several characteristics. Each was relatively infrequent (returning at intervals of decades to centuries) and had strong
exogenous forcing. The spatial patterns of a disturbance could be predicted based on topography if it had
a known directionality (e.g., prevailing wind direction) and a sheltering
effect. Although these large disturbances were spatially extensive, in
all of them the effects across the
landscape were heterogeneous. The
complex disturbance-initiated mosaic created a legacy that will influence the ecosystem well into the future, regardless of whether primary
or secondary succession was initiated. Indeed, although their effects
on plants were not as catastrophic as
suggested by first impressions, these
disturbances may be the dominant
765
frequency and intensity of hurricanes.
Past climate changes have altered fire
regimes (e.g., Clark 1990), and simulations suggest that the Yellowstone
landscape would be substantially altered by the more frequent fires associated with a warmer, drier climate
or by the less frequent but even larger
fires associated with a cooler, wetter
climate (Gardner et al. 1996). Understanding the response of disturbance regimes characterized by large,
infrequent events to climatic change
remains an important challenge.
Our comparison of the effects of
the eruption of Mount St. Helens,
the Yellowstone fires, and Hurricane Hugo suggests intriguing similarities and differences among these
large disturbances. Our understanding of the effects of large disturbances would benefit from comparisons with other types of disturbance.
For example, large, infrequent
flooding events in river systems
(e.g., Baker and Walford 1995) and
in near-coastal communities, such
as coral reefs and kelp forests, are
influenced by infrequent, severe
storm events (e.g., Dayton et al.
1992). Our analysis highlights the
extremely heterogeneous nature of
large-scale disturbances. Further
study of these and other large disturbances would enhance our understanding of how large-scale disturbances restructure landscapes
and influence succession.
forces influencing the structure of
these systems.
Considering all modes of reestablishment of vegetation, recovery rates
were highest following
Hurricane
Hugo and slowest following the eruption of Mount St. Helens. Mortality
induced by the disturbances was generally immediate and substantial for
the crown fire and volcano, whereas
hurricane-induced mortality was less
and lagged in time for as much as five
mechanisms
years. Recovery
appeared to be related more to disturbance severity than to disturbance
size per se. The greater survival of
dominant trees following Hurricane
Hugo likely was a major factor contributing to the more rapid recovery
after this disturbance. Propagule availability was generally not a factor controlling response to hurricane disturbance in Puerto Rico because of the
fine-grained heterogeneity of damage,
low mortality, rapid flowering and
fruiting, and an adequate soil seed
bank. Recovery rates were slowest
for the most severely disturbed areas
of Mount St. Helens. When recovery
was relatively rapid (as for the crown
fire and hurricane), the window of
opportunity for seedling establishment was short and pulsed. By contrast, when recovery was slow (as for
the volcano), seedling establishment
seemed to be a protracted, gradual
process.
The effect of disturbance size appears to be strongly influenced by
the abundance and spatial distribution of survivors and propagulesthat is, by disturbance severity. If
survival is high or the seed bank is
sufficient, disturbance size may be
for succesrelatively unimportant
sion. By contrast, if survival and
propagule availability are both low,
then disturbance size becomes important for colonization and for esand chance colonizatablishment,
tion events may send succession in
different directions.
Given the importance of largescale disturbances in structuring ecosystems, changes in these disturbance
regimes-for
example, as a result of
have
global climate change-could
substantial ecological implications.
Global climate change would not
affect the frequency of volcanic activity, but higher ocean surface tem-
This manuscript was improved by
comments from John H. Fitch, Ariel
E. Lugo, David J. Mladenoff, William H. Romme, John A. Wiens,
Rebecca Chasan, and two anonymous reviewers. Funding for the researchin Yellowstone National Park
was provided by the National Geographic Society (grant nrs 4284-90
and 5640-96) and the National Science Foundation (grant nrs BSR9016281 and BSR-9018381). Funding for the research at Mount St.
Helens was providedby the National
GeographicSociety (grant nr 520794). Fundingfor the studiesof Hurricane Hugo was provided by the National ScienceFoundation (grantnrs
BSR-9015961 and BSR-8811902).
This work was facilitated by a
peratures would increase both the
Global Change Fellowship from the
766
Acknowledgments
US Department of Energy to EWE.
We also acknowledge support from
the Ecological Research Division, Office of Health and Environmental
Research, US Department of Energy,
under contract DE-AC05-960R22464 with Lockheed Martin Energy Research, Inc. This article is
Publication nr 4668 of the Environmental Sciences Division, Oak Ridge
National Laboratory.
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